In some utility piping networks, the location of the pipes is mapped during the underground installation thereof so that they may be easily located and unearthed as required at subsequent times. Underground pipes may alternatively be located by using a conventional magnetometer moved along the surface of the ground for detecting changes in magnetic fields caused by ferrous pipes.
Some pipes are non-ferrous or non-metallic, such as plastic pipes, which cannot be magnetically detected when installed underground. To resolve this problem, various techniques are known for magnetically marking such pipes by imbedding magnetic particles therein during the manufacturing process thereof, or by using magnetic marking tapes which are buried in the ground along with the pipe for the subsequent magnetic detection thereof. Acoustic tomography and ground penetrating radar could also, in principal, be used for detecting underground plastic pipes, but background noise would make it difficult for accurately locating the buried pipes. These methods also require highly trained operators and would be relatively slow, labor intensive, and expensive.